Radio frequency (RF) power amplifiers (PAs) are used to produce output transmit signals by amplifying weak input signals in wireless devices, such as cellular telephone handsets. Many of these communication devices are configured to operate in different frequency bands for different communication systems. For example, current third generation (3G) cellular communication systems utilize a variety of different frequency bands above 1 GHz, such as, for example, 1920-1980 Mhz for WCDMA. Prior second generation (2G) cellular communication systems also utilize a variety of different frequency bands, such as, for example, 824-849 Mhz for GSM 800, 880-915 Mhz for GSME 900, 1710-1785 Mhz for DCS 1800, and 1850-1910 Mhz for PCS 1900.
To operate in multiple frequency bands and thereby multiple communication systems, cellular handset applications typically use wideband power amplifiers to amplify and output transmit signals. These cellular wideband power amplifiers exhibit high gain across a wide frequency range, enabling them to amplify multiple frequency bands. However, transmit signal gain and noise in certain frequency bands is detrimental to system performance, for example, in the cellular receive band or in the bands used for other communication systems, such as those used by Bluetooth transceivers and/or GPS (global positioning system) receivers. In cellular handset applications, the wideband power amplifiers are typically followed by duplexers that pass the desired range of frequencies for the selected frequency band and reject signals in frequencies outside the selected frequency band. A tunable narrow-band filter on the input signal to the power amplifier may also be used to minimize the unwanted noise contributions outside the selected frequency band.
FIG. 1A (Prior Art) is a diagram of an example gain response 102 for a wideband power amplifier (PA). The wideband gain response 102 provides high gain for a wide range of frequencies between a lower frequency corner (fL) and a higher frequency corner (fH). As depicted, this wide range of frequencies 108 includes two communication frequency bands (B1, B2). The first communication frequency band (B1) includes a range of frequencies 104 between a first low frequency (fL1) and a first high frequency (fH1). The second communication frequency band (B2) includes a range of frequencies 106 between a second low frequency (fL2) and a second high frequency (fH2). Because the wideband PA provides high gain across both frequency bands, frequencies within the non-selected band can be amplified in the PA output signal in addition to frequencies within the desired frequency band. For example, if the second band (B2) is used for transmission by the communication device, undesired transmit signals or noise that have frequencies within the first band (B1) will still be amplified and output by the wideband PA.
FIG. 1B (Prior Art) is a block diagram of an embodiment 150 for output circuitry used by a communication device including a wideband PA 152. As depicted, the wideband PA 152 receives a transmit signal (TX) 168 that has been passed through a tunable narrow band filter 166. This transmit signal (TX) 168 can be provided, for example, from a baseband (BB) processor and/or a transceiver (XCVR) 160 in a communication device. The wideband PA 152 depicted is configured to be used for two possible frequency bands of operation (B1, B2). The amplified output of the wideband PA 152 is provided to a switch (SW) 154. The switch (SW) 154 is controlled by a band select signal 170, which can also be provided by the BB processor and/or transceiver (XCVR) 160. Based upon the band select signal 170, the switch (SW) 154 sends the amplified output signal to a first duplexer 156 for the first band (B1) or to a second duplexer 158 for the second band (B2). As depicted, the first duplexer (B1) 156 is configured to pass frequencies within the first frequency band (B1) and reject other frequencies. Similarly, the second duplexer (B2) 158 is configured to pass frequencies within the second frequency band (B2) and reject other frequencies. The output transmit signals from the duplexers 156 and 158 are then provided to the antenna switch module (ASM) 162. The duplexers 156 and 158 are used for transmit and receive operations in a bi-directional communication device. In receive mode, duplexer 156 outputs a first receive signal (RX1) 157 that can be provided to the baseband processor or transceiver 160. Similarly, in receive mode, duplexer 158 outputs a second receive signal (RX2) 157 that can be provided to the baseband processor or transceiver 160. The ASM 162 can also be configured to receive the band select signal 170, and the band select signal 170 can be used by the ASM 162 to couple the proper duplexer to the antenna 164. The tunable narrow band filter 166 can also be configured to receive the band select signal 170, and the band select signal 170 can be used to tune the tunable narrow band filter 166 for a selected band.
It is noted that in addition to the first band (B1) and the second band (B2), the communication device can be configured for operation in additional frequency bands that utilize different transmit and/or receive circuitry than that depicted in FIG. 1B (Prior Art). For example, the communication device could be configured to have a global position system (GPS) receiver and/or a Bluetooth transceiver, if desired, that also utilize the same antenna 164.
One disadvantage of the output circuitry 150 of FIG. 1B (Prior Art) is the need for a tunable narrow band filter at the input to the wideband PA 152 to reduce energy in non-selected frequency bands. Regulations associated with many communication systems restrict the amount of out-of-band energy that can be transmitted by a communication device operating with the communication system. For example, transmitted energy within a non-selected first band (B1) may need to be significantly less than transmitted energy within a selected second band (B2). Thus, as depicted in FIG. 1B, the narrow band filter at the input to the PA is used to provide the necessary out-of-band rejection needed to satisfy the regulatory requirements. Even if regulatory requirements are satisfied, the additional rejection is needed to improve the ability of the system to detect very weak receiver signals in the non-selected frequency bands, such as GPS or Bluetooth. The narrow band filter, however, raises the cost and size requirements for communication devices configured to operate in multiple frequency bands.